About wide vs. small gamuts, 8 vs. 16-bit &sRGB vs. Adobe'98

Started Apr 3, 2003 | Discussions thread
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Magne Nilsen
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About wide vs. small gamuts, 8 vs. 16-bit &sRGB vs. Adobe'98
Apr 3, 2003

(This is an early preview of an article that soon will appear on the http://www.etcetera.cc site)

With the many discussion appearing lately about colors, color spaces, 8-bit vs. 16-bit and sRGB vs. Adobe RGB (1998), there seem to be as many preferences as there are combinations, and most proponents of one versus the other often seems more confused than the other. I have a feeling that some perspective could be needed. It is not too hard to understand most of this, so...

This is an honest attempt to set some basic terms straight. The accompanying illustrations are made to perceptually convey what is discussed, and are not meant to be scrutinized by their byte values. They are for 'your eyes only'.

Let's simplify a little. The example above shows two gamuts. A gamut is all the colors that can be reproduced or captured by a device. The number of bits used to hold the gamut dictates the number of steps that each color can contain. Many bits will give room for many subtle variations or tones per color. The width of the gamut puts constraints upon, or limits the saturation level of colors inside the gamut. A wide gamut can hold very saturated colors, while a small gamut will sacrifice those most vivid colors. As long as the gamut is, and stays, both wide and has many bits to represent the color values, few problems should occur in handling images. As you see from the illustration above, the "large gamut" version has more saturated blues, greens, reds, yellows etc. Why would anyone consider the "small gamut" version at all?

These are the same gamuts as in the first image, but this time represented with fewer bits. The large gamut version still has more saturated colors, but now there is a price to pay! The "few-bits large gamut" has sacrificed many of the more subtle tonalities to give room for the more saturated and neon-like colors. Which finite set of the colors above do you think would be the best to accurately describe human skin, like in a portrait? Or - a sunset? Or - a small lake with a green forest around?

These illustrations are much worse than the difference between 8-bit vs. 16-bit, and sRGB vs. Adobe RGB (1998), but this is still the dilemma you are going to face in the real world of printers and output devices. Sooner or later.

At some stage the "many-bits large gamut" image must be color-wise transformed into a "not-so-wide" and "not-so-many-bits" gamut, representing a display, a web page or a printer. Printer gamuts are actually mostly smaller than sRGB.

At some stage you will be doing things in 8-bit software as well. Photoshop has some limitations on the available functionality for 16-bit images, but I would still advice all to switch to 16-bit as soon as possible, and stay there as long as possible when editing images. When you go from “wide gamut 16-bit” to “small gamut 8-bit” you should know what will happen, or you could be in for a surprise, or even worse, your customer/client/uncle could be in for a color-wise surprise.

As an example on how bad things can get, look at the last illustration.

Applying an S-curve as strong as the one above onto the prior shown “few-bits large gamut” image will result in a reduction from 400 finite colors down to 300. 100 colors (25% of the gamut!) will be gone, and many colors will show up as identical in the image. The same thing would happen if we were to show a “few-bits huge gamut” image on your “not so huge gamut” display. You would not be able to discern many of the colors from each other at all.

My intention with this is not to scare anyone away from using a wide gamut working space to hold or edit their images. My advice would be to check up what exact colors you are getting in Adobe RGB 1998 that are not present in sRGB and vice versa. I think you could be surprised.

16-bit editing is almost without an exception an advantage. Especially if you are going to use a wider gamut like Adobe 1998 RGB (or maybe even wider?). You will have a much bigger latitude in you editing operations before you start to experience ugly 8-bit problems like banding of neighboring colors, or posterization problems in the shadows or in skies. Switching to, and staying in 16-bit as long as possible, is almost always an advantage for your images. I think 16-bit is much more important than using Adobe RGB 1998 as a working space, but I constantly find professionals that have heard that Adobe RGB 1998 is so good, but they still do all their editing in 8-bit, and wonder why they have banding and posterization problems. Most of them believe that this would be even worse if they had sRGB as their working space, while the opposite is true. I hope that became clear with the prior illustrations.

Ah well, 16-bit is “better” than 8-bit then, but wide gamut is not “better” than small gamut, although the words wide and small for many would indicate so. It really depends. On the destination target, on the software available, on your skills and knowledge, on your input device/monitor/output devices level of quality, calibration and exactness, and on a few physical truths like bandwidth, memory, processor and hard disk size and quality. In many cases it is actually an advantage to be working in a “not-so-wide” gamut like sRGB. You get more subtle transitions at the cost of some “neons”, and you will be much less surprised or disappointed when transferring your images to the web, the printer, the lab, the clients or your friends.

~~~~
Magne

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